Crosslinked polymers typified by crosslinked (meth)acrylic polymers are used for various applications, such as various coating compositions (as a binder or a filler), an adhesive component, an electronics-related material, a controlled release mechanism of a medicinal composition, and a resin composite material. While crosslinked polymers can be prepared, for example, in the form of polymer particles via a polymerization method of a monomer composition containing polyfunctional monomers, such as emulsion polymerization, dispersion polymerization, suspension polymerization, precipitation polymerization, and solution polymerization (see, for example, Patent Document 1), it has problematically not been possible to increase the solid content (concentration) of a crosslinked polymer in a resulting crosslinked polymer solution especially in preparation by solution polymerization. Namely, there was a problem that an attempt to achieve a high solid content results in the occurrence of gelation, which inhibits obtaining a desired crosslinked polymer (polymer particles) or inhibits use for a desired application.
As a means for solving the above-mentioned problem, Patent Document 2 discloses that a reaction material comprising a monomer, a crosslinking agent, and a polymerization initiator is added to a heated solvent by using a pulse addition method in a method for preparing crosslinked polymer nanoparticles made of a (meth)acrylic crosslinked polymer by solution polymerization (Claim 1, Examples 1 and 2, etc.). The pulse addition method is a method in which a time zone during which part of the reaction material is added and a time zone during which the reaction material is not added are repeated until the addition of the whole amount of the reaction product is completed.
Patent Document 3 discloses a resin prepared using a monomer having plural α,β-unsaturated double bonds in a high solid concentration. The resin disclosed in Patent Document 3, however, cannot afford a sufficient effect with respect to weatherability when being used as a coating because the resin does not have a satisfactorily high molecular weight. Patent reference 3 causes a problem that a resin having a satisfactorily high molecular weight cannot be obtained because a reaction is conducted within a temperature region such that the half-life of a radical polymerization initiator is short.
Patent document 4 discloses a resin prepared using a monomer having plural α,β-unsaturated double bonds in silicone oil. However, the resin obtained by reacting the monomer in the silicone oil is unusable for coating compositions as it is, a problem such as a repelling occurs when the resin is incorporated. A step of removing the silicone oil to be used as a medium is disclosed, but performing this step results in an increase in steps and in costs.